Bearing steel and method of manufacturing the same

ABSTRACT

A high-quality bearing steel contains 0.70 to 1.10% by weight of C, 0.15 to 1.60% by weight of Si, 0.15 to 1.15% by weight of Mn, 0.010% by weight or less of P, 0.002% by weight or less of S, 0.50 to 1.60% by weight of Cr, 0.015% by weight or less of Al, 0.0050% by weight or less of N, 0.0006% by weight or less of O, 0.0015% by weight or less of Ti, and the remainder of Fe together with impurities. The steel can further contain 0.05 to 0.50% by weight of Mo or a member or members selected from a group consisting of 0.05 to 0.30% by weight of V and 0.05 to 0.30% by weight of Nb. the bearing steel has an excellent durability life and cold workability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-quality bearing steel havingexcellent durability life and cold workability and used for ballbearings, roller bearings and the like, and a manufacturing methodtherefor.

2. Description of the Prior Art

Bearing steel must satisfy various properties including those relatingto durability life, impact fatigue, cold workability and the like. Inparticular, durability life is becoming increasingly important withtrends toward heavier loads and higher speed together with therequirements of higher performance in industrial machinery and vehicles.Studies are being made to develop a steel having higher durability life.

In order to improve durability life, a conventional method was proposedwherein the O content of the steel is decreased to reduce oxideinclusions such as Al₂ O₃. In some applications, another conventionalmethod was proposed which uses a special smelting method such as VAR orESR so as to control the solid texture and to reduce the amount ofnon-metallic inclusions.

However, in the conventional method of simply reducing the O content ofthe steel, a satisfactory long durability life cannot be obtained insome applications. Again, the latter method is costly and not suitableto mass-production.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of this and isbased on various studies made on the influence of various alloyingelements on the durability life of a resultant bearing steel. Based onsuch studies, it was found that alumina oxide among oxide inclusionsproduces large inclusions and considerably reduces the durability life.When oxide inclusions are very small, a trace amount of sulfideinclusion reduces the durability life while Ti forms a carbo-nitride,and a mere trace considerably reduces the durability life. Impuritiessuch as P or N also adversely affect the durability life.

It is a primary object of the present invention to provide ahigh-quality bearing steel which has an excellent durability life: arated life (B10) three times and an average life (B50) six times thoseof conventional steel, and which therefore has a durability equivalentto that of an ESR material.

It is another object of the present invention to provide a high-qualitybearing steel which has a very small amount of oxide inclusions andwhich has an improved cold workability.

In order to achieve the above and other objects of the presentinvention, there is provided a high-quality bearing steel wherein theamounts of alloying elements in the steel are selected to be 0.70 to1.10% by weight for C, 0.15 to 1.60% by weight for Si, 0.15 to 1.15% byweight for Mn, and 0.50 to 1.60% by weight for Cr, and an O content is0.0006% which is the minimum O content that can be achieved with thecurrent vacuum degassing scouring technique. Raw materials are selectedbased on strict standards to reduce the amounts of impurities which ifintroduced form inclusions reducing durability life. The amount of Ti isset to be 0.0015% by weight or less and the amount of Al is set to be0.015% by weight or less, and the amount of S is reduced to 0.002% byweight or less which is considerably smaller than that in conventionalsteel. The amounts of impurity elements are reduced, i.e., the amount ofP is reduced to 0.010% by weight or less and the amount of N is reducedto 0.0050% by weight or less so as to reduce the amount of non-metallicinclusions in the steel to 0.010% by area or less. The average size ofnon-metallic inclusions is reduced to 15 μm or less.

In a method of manufacturing a highly pure bearing steel having thecomponents described above according to the present invention, good rawmaterials of steel are selected, and after oxidation scouring in anelectric furnace poured into a ladle. The smelted steel is subjected todephosphorization during or after pouring into the ladle. The oxide slagon the smelted steel is absorbed and removed with a vacuum slag cleaner.A highly basic slag having a basicity of 3 or more (a reducing slaghaving an excellent desulfurization property such that FeO+MnO≦0.5% (byweight) and CaO/SiO₂ /Al₂ O₃ =0.3 to 0.4) is prepared by an electricfurnace. Reducing refinement is performed to reduce the amount of S to0.002% by weight or less and the amount of O to 0.0020% by weight orless and to reduce the amount of Ti while bath temperature iscontrolled, an inert gas is introduced through double porous bricks, andthe smelted steel is agitated. Subsequently, vacuum degassing isperformed by a circulating vacuum degassing apparatus such that vigorouscirculating is performed during 2/3 of the total treatment time whileweak circulating is performed during 1/3 of the total treatment time,thereby further reducing the amounts of O, N and H. Reducing refinementis then performed by weakly agitating the smelted steel in a reducingatmosphere at a pressure higher than normal pressure to allow minuteinclusions to float and be removed. Finally, sealed casting is performedto provide the highly pure bearing steel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first steel according to the present invention consists essentially of0.70 to 1.10% by weight of C, 0.15 to 1.60% by weight of Si, 0.15 to1.15% by weight of Mn, 0.010% by weight or less of P, 0.002% by weightor less of S, 0.50 to 1.60% by weight of Cr, 0.015% by weight or less ofAl, 0.0006% by weight or less of O, 0.0050% by weight or less of N, and0.0015% by weight or less of Ti, and the remainder being Fe and impurityelements. A second steel according to the present invention consistsessentially of 0.05 to 0.50% by weight of Mo and other components of thefirst steel and has improved hardenability. A third steel according tothe present invention consists essentially of one of V and Nb or both by0.05 to 0.30% each by weight in addition to the components of the firststeel so as to improve wear resistance. According to the method ofmanufacturing a bearing steel of the present invention, when the steelis prepared, slag on the smelted steel poured from a smelting furnace toa separate container (ladle) is absorbed and removed with a vacuum slagcleaner. Then, in the presence of a highly basic slag, reducingrefinement is performed under conditions of strong agitation and bathtemperature control. Vacuum degassing is then performed by strong andweak circulating by a circulating vacuum degassing apparatus. Nextreducing refinement is performed under weak agitation in a reducingatmosphere at normal pressure.

The reasons for limiting the upper and/or lower limits of the contentsof the respective components of the steel according to the presentinvention will be described below.

Carbon is an important element which must be included to achieve ahardness H_(R) C of 60 or higher required for a bearing steel. Carbonmust be present in the amount of 0.70% by weight or more in order toachieve such requirement. However, when C is contained in the amountexceeding 1.10% by weight, large carbides tend to form and durabilitylife and impact fatigue are reduced. For this reason, the upper limit ofC content in steel is set to be 1.10% by weight.

Silicon is an element necessary to improve deoxidation property andhardenability as well as to improve durability life and impact fatigueproperties. Silicon must be contained in the amount of 0.15% by weightor more. However, when Si is contained in the amount of 1.60% by weightor more, contact rolling fatigue life (hereinafter mentioned as "rollinglife") characteristics are degraded. For this reason, the upper limit ofthe Si content in the steel is set to be 1.60% by weight.

Manganese is an element for improving deoxidation property andhardenability and must be contained in the amount of 0.15% by weight ormore. However, if Mn is contained in a larger amount, the addition doesnot provide better effect and also produce MnS which degrades rollinglife. Therefore, the upper limit of manganese contained in the steel isset to be 1.15% by weight.

Chromium is also an element for improving hardenability and forfacilitating easy formation of spherical carbides. It is necessary tocontain Cr in the amount of 0.50% by weight or more in order to attainsuch improvements. However, when Cr is contained in the amount exceedingthis upper limit, the carbides become too large to degrade the cuttingperformance. Thus, the upper limit of the Cr content in the steel is setto be 1.60% by weight.

Phosphorus is an element which reduces rolling life and toughness andmust be restricted to the smallest amount possible. The upper limit of Pcontained in the steel is set to be 0.010% by weight.

Sulfur is an element which forms a sulfide inclusion with Mn andconsiderably degrades the durability life. The primary objects of thepresent invention are to reduce the sulfide inclusions and toconsiderably improve the durability life. The amount of S contained inthe steel must be precisely controlled and its upper limit is set to be0.002% by weight.

Aluminum is an element which produces a hard oxide inclusion such as Al₂O₃ which degrades steel cutting performance and rolling lifecharacteristics. The amount of Al contained in the steel therefore mustbe reduced to a minimum, and its upper limit is set to be 0.015% byweight.

Oxygen is an element which produces oxide inclusions such as Al₂ O₃ orSiO₂ and considerably degrades the rolling life characteristic.According to the present invention, the amount of non-metallicinclusions is reduced considerably and the inclusion size is controlledso that the rolling life characteristic is improved. Thus, the amount ofO contained in the steel must be strictly controlled. The upper limit ofO contained in the steel is set to be 0.0006% by weight which is aminimum amount achievable with a currently available vacuum degassingscouring technique.

Nitrogen is an element which forms a nitride such as TiN and degradesthe rolling life characteristic. The N content in the steel must be setto a minimum, and its upper limit is set to be 0.0050% by weight.

Titanium is an element which remains in the steel in the form a TiNinclusion. When Ti is contained in a large amount in the steel, it formslarge inclusions and considerably degrades the rolling lifecharacteristic. Therefore, the amount of Ti contained in the steel mustbe reduced to a minimum and its upper limit is set to be 0.0015% byweight.

Molybdenum is an element for improving the steel hardenability. When Mois contained in a small amount in the steel, hardenability is improved.According to the present invention, Mo is contained in the amount of0.05% by weight or more, if necessary. However, since Mo is an expensiveelement and addition thereof in an amount exceeding 0.50% by weight doesnot provide an equivalent good result, the upper limit of Mo content inthe steel is set to be 0.50% by weight.

Vanadium and niobium are elements for improving steel strength andtoughness by forming carbo-nitride. In the present invention, it isnecessary to contain V and/or Nb in the steel in the amounts of 0.05% byweight or more, respectively, to obtain desired effects, if it isdesired. However, even if these elements are contained in amountsexceeding this upper limit, the effect of addition is small. Therefore,the upper limits for these elements are set to be 0.30% by weight.

The characteristic features of the steel of the present invention willbe described by way of examples in comparison with those of conventionalsteels.

Table 1 shows the chemical components of sample steel.

                                      TABLE 1                                     __________________________________________________________________________    Chemical Composition (wt %)                                                   C    Si Mn P  S  Cu Ni Cr Al  O   N   Ti  Mo V  Nb                            __________________________________________________________________________    A 0.98                                                                             0.28                                                                             0.47                                                                             0.016                                                                            0.012                                                                            0.13                                                                             0.06                                                                             1.46                                                                             0.024                                                                             0.0012                                                                            0.0051                                                                            0.0030                                  B 0.96                                                                             0.60                                                                             1.01                                                                             0.014                                                                            0.013                                                                            0.14                                                                             0.06                                                                             1.11                                                                             0.023                                                                             0.0013                                                                            0.0045                                                                            0.0029                                  C 0.96                                                                             0.27                                                                             0.49                                                                             0.010                                                                            0.007                                                                            0.08                                                                             0.08                                                                             1.47                                                                             0.0018                                                                            0.0006                                                                            0.0052                                                                            0.0032                                  D 0.98                                                                             0.28                                                                             0.46                                                                             0.012                                                                            0.003                                                                            0.05                                                                             0.06                                                                             1.49                                                                             0.0015                                                                            0.0010                                                                            0.0047                                                                            0.0017                                  E 0.77                                                                             0.28                                                                             0.46                                                                             0.010                                                                            0.009                                                                            0.09                                                                             0.05                                                                             1.46                                                                             0.0019                                                                            0.0007                                                                            0.0049                                                                            0.0012                                  F 0.82                                                                             0.30                                                                             0.47                                                                             0.007                                                                            0.002                                                                            0.05                                                                             0.08                                                                             1.51                                                                             0.0013                                                                            0.0005                                                                            0.0049                                                                            0.0013                                  G 0.82                                                                             0.67                                                                             1.03                                                                             0.009                                                                            0.002                                                                            0.08                                                                             0.10                                                                             1.16                                                                             0.0015                                                                            0.0005                                                                            0.0047                                                                            0.0012                                  H 0.97                                                                             0.28                                                                             0.47                                                                             0.009                                                                            0.002                                                                            0.07                                                                             0.09                                                                             1.47                                                                             0.0012                                                                            0.0005                                                                            0.0045                                                                            0.0014                                  J 0.98                                                                             0.62                                                                             0.96                                                                             0.007                                                                            0.002                                                                            0.04                                                                             0.08                                                                             1.07                                                                             0.0010                                                                            0.0005                                                                            0.0047                                                                            0.0011                                  K 0.97                                                                             0.27                                                                             0.46                                                                             0.008                                                                            0.002                                                                            0.05                                                                             0.06                                                                             1.49                                                                             0.0012                                                                            0.0005                                                                            0.0042                                                                            0.0011                                                                            0.15                                L 0.97                                                                             0.29                                                                             0.51                                                                             0.008                                                                            0.002                                                                            0.09                                                                             0.08                                                                             1.46                                                                             0.003                                                                             0.0005                                                                            0.0042                                                                            0.0012 0.11                             M 0.77                                                                             0.29                                                                             0.47                                                                             0.007                                                                            0.002                                                                            0.05                                                                             0.08                                                                             1.46                                                                             0.0012                                                                            0.0005                                                                            0.0045                                                                            0.0010    0.13                          N 0.96                                                                             0.60                                                                             1.05                                                                             0.009                                                                            0.002                                                                            0.12                                                                             0.05                                                                             1.15                                                                             0.0012                                                                            0.0005                                                                            0.0049                                                                            0.0011 0.10                                                                             0.12                          __________________________________________________________________________

In Table 1, steels A and B are conventional steels (steel A: SUJ2; steelB: SUJ3) steels C to E are comparative steels, steel D is prepared byESR, and steels F to N are steels of the present invention.

Table 2 shows the results of an experiment for determining the amountand average length of non-metallic inclusions, durability life, and coldworkability for the sample steels presented in Table 1 when these samplesteels were hardened by oil cooling under conditions of 850° C.×30minutes, tempered under conditions of 170° C.×90 minutes, and then aircooled.

The amount and average length of non-metallic inclusions were tested bycutting rolled bars of 65 mm diameter from the steel subjected to theabove treatments. The amount and average length of non-metallicinclusions in each steel are represented comparatively with the lengthof non-metallic inclusions of steel D as an ESR material which is hypothsize unit. The durability life was tested using a Mori-type durabilitylife tester. Sample pieces having an outer diameter of 65 mm, an innerdiameter of 18 mm, and a thickness of 10 mm were prepared. Thedurability life is also represented comparatively with the length ofnon-metallic inclusions of steel D. The cold workability was examined interms of cracking rate at an upsetting rate of 75% (20 mm diameter and30 mm length).

                  TABLE 2                                                         ______________________________________                                        Non-metallic                                                                  Inclusion          Durability Cold                                                         Average   Life       Workability                                 Amount JIS   Length    B.sub.10                                                                             B.sub.50                                                                            Cracking                                  (A + B + C)  (μm)   Life   Life  Rate (%)                                  ______________________________________                                        A    3.4         2.0       0.37 0.15  18                                      B    3.2         2.0       0.36 0.15  50                                      C    2.4         1.6       0.39 0.16  11                                      D    1.0         1.0       1.0  1.0   0                                       E    2.3         1.6       0.51 0.18  15                                      F    0.25        1.0       1.29 1.10  0                                       G    0.25        1.0       1.26 1.09  0                                       H    0.25        1.0       1.07 1.01  0                                       J    0.25        1.0       1.05 1.0   0                                       K    0.50        1.0       1.01 1.0   0                                       L    0.50        1.0       1.06 1.0   0                                       M    0.50        1.0       1.06 1.0   0                                       N    0.50        1.0       1.0  1.0   0                                       ______________________________________                                    

As can be seen from Table 2, the non-metallic inclusions of theconventional steels A and B have amounts which are three times those ofthe steel D as an ESR material having a controlled solid texture andalso have average lengths which are twice those of the steel D. Theconventional steels A and B have the durability life 1/3 that of steel Din terms of rated life and of about 1/6 that of steel D in terms ofaverage life. As for the cold workability, the steels A and B have highcracking rates. Thus, the steels A and B are inferior in terms ofnon-metallic inclusions, durability life, and cold workability.

The steel C as a comparative steel is slightly improved as to amount andaverage length of non-metallic inclusions compared with the conventionalsteels due to the low S and O contents. However, the steel D has theamount of non-metallic inclusions which is 2.4 times that of the steelD, and the average length of non-metallic inclusions which is 1.6 timesthat of the steel D. The steel C has a durability equivalent to theconventional steels A and B. The steel E as another comparative steel isalso inferior in terms of non-metallic inclusion amount and averagelength, and durability life, as in the case of the steel C.

In contrast to this, in the steels F to N of the present invention,since the O content is 0.0006% by weight or less and the S content is0.002% by weight or less and since the Ti, Al and N contents are reducedto a minimum, they have an amount of non-metallic inclusions which is1/4 that of the steel D as an ESR material, an average length of thenon-metallic inclusions equivalent to the steel D, a durability lifeequivalent to that of the steel D in terms of rated and average lives,and a better cold workability than those of the conventional steels Aand B. In this manner, the steels F to N according to the presentinvention exhibit better performance as to the non-metallic inclusions,and equivalent durability life and cold workability compared to thesteel D as an ESR material which has an improved internal quality due tocontrolled solid texture.

In the steel according to the present invention, the S content is as lowas 0.002% by weight or less. However, since the amount of oxideinclusions is considerably reduced, machinability equivalent to theconventional steels is obtained.

As can be seen from the above, in the steel of the present invention, C,Si, Mn and Cr are contained in proper amounts, and P, S, O and Ticontents are reduced to a minimum so as to reduce the amounts of oxideand sulfide inclusions in the steel. Thus, the steel according to thepresent invention has an excellent durability life, a rated life threetimes and an average life six times those of conventional steels, andalso has an excellent cold workability. The present invention thusprovides a high-quality bearing steel suitable for use in ball bearingsor roller bearings in high-load and high-speed industrial machinery orvehicles, and a method of manufacturing thereof.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A bearing steel of improved durability life,consisting essentially of, by weight:0.70-1.10% carbon, 0.15-1.60% Si,0.15-1.15% Mn, 0.50-1.60% Cr, not more than 0.010% P, not more than0.002% S, not more than 0.015% Al, not more than 0.0006% O, not morethan 0.0050% N, and not more than 0.0015% Ti, the remainder being Fetogether with impurities, said steel having a reduced amount ofnon-metallic inclusions of 0.010% by area or less with the average sizeof the inclusions being no more than 15 microns.
 2. The bearing steelaccording to claim 1, wherein the steel consists essentially of, byweight, 0.70-0.85% carbon, 0.15-0.35% silicon, 0.15-0.50% manganese,1.30-1.60% chromium, not more than 0.010% phosphorus, not more than0.002% sulfur, not more than 0.015% aluminum, not more than 0.0006%oxygen, not more than 0.0050% nitrogen and not more than 0.0015%titanium, the remainder being iron together with impurities.
 3. Thebearing steel according to claim 1, wherein the steel consistsessentially of, by weight, 0.70-0.85% carbon, 1.20-1.60% silicon,0.15-0.50% manganese, 1.30-1.60% chromium, not more than 0.010%phosphorus, not more than 0.002% sulfur, not more than 0.015% aluminum,not more than 0.0006% oxygen, not more than 0.0050% nitrogen and notmore than 0.0015% titanium, the remainder being iron together withimpurities.
 4. The bearing steel according to claim 1, wherein the steelconsists essentially of, by weight, 0.95-1.10% carbon, 1.20-1.60%silicon, 0.15-0.50% manganese, 1.30-1.60% chromium, not more than 0.010%phosphorus, not more than 0.002% sulfur, not more than 0.015% aluminum,not more than 0.0006% oxygen, not more than 0.0050% nitrogen and notmore than 0.0015% titanium, the remainder being iron together withimpurities.
 5. The bearing steel according to claim 1, wherein the steelconsists essentially of, by weight, 0.95-1.10% carbon, 0.15-0.35%silicon, 0.15-0.50% manganese, 1.30-1.60% chromium, not more than 0.010%phosphorus, not more than 0.002% sulfur, not more than 0.015% aluminum,not more than 0.0006% oxygen, not more than 0.0050% nitrogen and notmore than 0.0015% titanium, the remainder being iron together withimpurities.
 6. A bearing steel consisting essentially of, by weight,0.70-1.10% carbon, 0.15-1.60% silicon, 0.15-1.15% manganese, 0.50-1.60%chromium, not more than 0.010% phosphorus, not more than 0.002% sulfur,not more than 0.015% aluminum, not more than 0.0006% oxygen, not morethan 0.0050% nitrogen, not more than 0.0015% titanium, and 0.15-0.50%molybdenum, the remainder being iron together with impurities.
 7. Thebearing steel according to claim 6, wherein the steel consistsessentially of, by weight, 0.70-0.85% carbon, 0.15-0.35% silicon,0.15-0.50% manganese, 1.30-1.60% chromium, not more than 0.010%phosphorus, not more than 0.002% sulfur, not more than 0.015% aluminum,not more than 0.0006% oxygen, not more than 0.0050% nitrogen, not morethan 0.0015% titanium and 0.15-0.30% molybdenum, the remainder beingiron together with impurities.
 8. The bearing steel according to claim6, wherein the steel consists essentially of, by weight, 0.95-1.10%carbon, 0.15-0.35% silicon, 0.15-0.50% manganese, 1.30-1.60% chromium,not more than 0.010% phosphorus, not more than 0.002% sulfur, not morethan 0.015% aluminum, not more than 0.0006% oxygen, not more than0.0050% nitrogen, not more than 0.0015% titanium and 0.15-0.30%molybdenum, the remainder being iron together with impurities.
 9. Abearing steel consisting essentially of, by weight, 0.70-1.10% carbon,0.15-1.60% silicon, 0.15-1.15% manganese, 0.50-1.60% chromium, not morethan 0.010% phosphorus, not more than 0.002% sulfur, not more than0.015% aluminum, not more than 0.0006% oxygen, not more than 0.0050%nitrogen, not more than 0.0015% titanium, and at least one memberselected from the group consisting of 0.05-0.30% vanadium and 0.05-0.30%niobium, the remainder being iron together with impurities.
 10. Thebearing steel according to claim 9, wherein the steel consistsessentially of, by weight, 0.70-0.85% carbon, 0.15-0.35% silicon,0.15-0.50% manganese, 1.30-1.60% chromium, not more than 0.010%phosphorus, not more than 0.002% sulfur, not more than 0.015% aluminum,not more than 0.0006% oxygen, not more than 0.0050% nitrogen, not morethan 0.0015% titanium and at least one member selected from the groupconsisting of 0.05-0.30% vanadium and 0.05-0.30% niobium, the remainderbeing iron together with impurities.
 11. The bearing steel according toclaim 9, wherein the steel consists essentially of, by weight,0.95-1.10% carbon, 0.15-0.35% silicon, 0.15-0.50% manganese, 1.30-1.60%chromium, not more than 0.010% phosphorus, not more than 0.002% sulfur,not more than 0.015% aluminum, not more than 0.0006% oxygen, not morethan 0.0050% nitrogen, not more than 0.0015% titanium, and at least onemember selected from the group consisting of 0.05-0.30% vanadium and0.05-0.30% niobium, the remainder being iron together with impurities.12. A bearing device of improved durabliity life composed of alloysteel, which consists essentially of by weight: 0.70-1.10% carbon,0.15-1.60% silicon, 0.15-1.15% manganese, 0.50-1.60% chromium, not morethan 0.010% phosphorus, not more than 0.0002% sulfur, not more than0.015% aluminum, not more than 0.0006% oxygen, not more than 0.0050%nitrogen and not more than 0.0015% titanium, the remainder being irontogether with impurities, said steel having a reduced amount ofnon-metallic inclusions of 0.010% by area or less with the average sizeof the inclusions being no more than 15 microns.
 13. The bearing deviceaccording to claim 12, wherein the steel consists essentially of, byweight: 0.70-0.85% carbon, 0.15-0.35% silicon, 0.15-0.50% manganese,1.30-1.60% chromium, not more than 0.010% phosphorus, not more than0.002% sulfur, not more than 0.015% aluminum, not more than 0.0006%oxygen, not more than 0.0050% nitrogen and not more than 0.0015%titanium, the remainder being irom together with impurities.
 14. Thebearing device according to claim 12, wherein the steel consistsessentially of, by weight: 0.70-0.85% carbon, 1.20-1.60% silicon,0.15-0.50% manganese, 1.30-1.60% chromium, not more than 0.010%phosphorus, not more than 0.002% sulfur, not more than 0.015% aluminum,not more than 0.0006% oxygen, not more than 0.0050% nitrogen and notmore than 0.0015% titanium, the remainder being iron together withimpurities.
 15. The bearing device according to claim 12, wherein thesteel consists essentially of, by weight: 0.95-1.10% carbon, 1.20-1.60%silicon, 0.15-0.50% manganese, 1.30-1.60% chromium, not more than 0.010%phosphorus, not more than 0.002% sulfur, not more than 0.015% aluminum,not more than 0.0006% oxygen, not more than 0.0050% nitrogen and notmore than 0.0015% titanium, the remainder being iron together withimpurities.
 16. The bearing device according to claim 12, wherein thesteel consists essentially of, by weight: 0.95-1.10% carbon, 0.15-0.35%silicon, 0.15-0.50% manganese, 1.30-1.60% chromium, not more than 0.010%phosphorus, not more than 0.002% sulfur, not more than 0.15% aluminum,not more than 0.0006% oxygen, not more than 0.0050% nitrogen and notmore than 0.0015% titanium, the remainder being iron together withimpurities.
 17. A bearing device of improved durability life composed ofalloy steel which consists essentially of, by weight: 0.70-1.10% carbon,0.15-1.60% silicon, 0.15-1.15% manganese, 0.50-1.60% chromium, not morethan 0.010% phosphorus, not more than 0.002% sulfur, not more than0.015% aluminum, not more than 0.0006% oxygen, not more than 0.0050%nitrogen, not more than 0.0015% titanium, and 0.15-0.50% molybdenum, theremainder being iron together with impurities, said steel having areduced amount of non-metallic inclusions of 0.010% by area or less withthe average size of the inclusions being no more than 15 microns. 18.The bearing device according to claim 17, wherein the steel consistsessentially of, by weight: 0.70-0.85% carbon, 0.15-0.35% silicon,0.15-0.50% manganese, 1.30-1.60% chromium, not more than 0.010%phosphorus, not more than 0.002% sulfur, not more than 0.015% aluminum,not more than 0.0006% oxygen, not more than 0.0050% nitrogen, not morethan 0.0015% titanium and 0.15-0.30% molybdenum, the remainder beingiron together with impurities.
 19. The bearing device according to claim17, wherein the steel consists essentially of, by weight: 0.95-1.10%carbon, 0.15-0.35% silicon, 0.15-0.50% manganese, 1.30-1.60% chromium,not more than 0.010% phosphorus, not more than 0.002% sulfur, not morethan 0.015% aluminum, not more than 0.0006% oxygen, not more than0.0050% nitrogen, not more than 0.0015% titanium and 0.15-0.30%molybdenum, the remainder being iron together with impurities.
 20. Abearing device of improved durability life composed of alloy steel,which consists of, by weight: 0.70-1.10% carbon, 0.15-1.60% silicon,0.15-1.15% manganese, 0.50-1.60% chromium, not more than 0.010%phosphorus, not more than 0.002% sulfur, not more than 0.015% aluminum,not more than 0.0006% oxygen, not more than 0.0050% nitrogen, not morethan 0.0015% titanium, nad a least one member selected from the groupconsisting of 0.05-0.30% vanadium and 0.05-0.30% niobium, the remainderbeing iron together with impurities, said steel having a reduced amountof non-metallic inclusions of 0.010% by area or less with the averagesize of the inclusions being no more than 15 microns.
 21. The bearingdevice according to claim 20, wherein the steel consists essentially of,by weight: 0.70-0.85% carbon, 0.15-0.35% silicon, 0.15-0.50% manganese,1.30-1.60% chromium, not more than 0.010% phosphorus, not more than0.002% sulfur, not more than 0.015% aluminum, not more than 0.0006%oxygen, not more than 0.0050% nitrogen, not more than 0.0015% titaniumand at least one member selected from the group consisting of 0.05-0.30%vanadium and 0.05-0.30% niobium, the remainder being iron together withimpurities.
 22. The bearing device according to claim 20, wherein thesteel consists essentially of, by weight: 0.95-1.10% carbon, 0.15-0.35%silicon, 0.15-0.50% manganese, 1.30-1.60% chromium, not more than 0.010%phosphorus, not more than 0.002% sulfur, not more than 0.015% aluminum,not sore than 0.0006% oxygen, not more than 0.0050% nitrogen, not morethan 0.0015% titanium, and at least one member selected from the groupconsisting of 0.05-0.30% vanadium and 0.05-0.30% niobium, the remainderbeing iron together with impurities.
 23. A bearing steel of improveddurability life, as well as being sufficiently able to bear heavierloads and higher speeds when subjecting the steel to these useconditions, consisting essentially of, by weight: 0.70-1.10% carbon,0.15-1.60% Si, 0.15-1.15% Mn 0.50-1.60% Cr, not more than 0.010% P, notmore than 0.002% S, not more than 0.015% Al, not more than 0.0006%oxygen, not more than 0.0050% N and not more than 0.0015% Ti, with theremainder being Fe and impurities, said steel having non-metallicinclusions in an amount of no more than 0.010% by area, and saidnon-metallic inclusions having an average size of no more than 15 μm.